A model and method have been developed for assessing quantitative aspects of 25-hydroxyvitamin D3 in man, through the use of H3-250HD3 injection followed by compartmental analysis. Present studies include definition of normal parameters, as well as the qualitative disturbances associated with hepatabiliary or intestinal disorders. BIBLIOGRAPHIC REFERENCE: S.B. Arnaud, R.S. Goldsmith, P.W. Lambert, and V.L.W. Go. 25-Hydroxyvitamin D3: Evidence of an Enterohepatic Circulation in Man. Proc Soc Exper Biol and Med 149:570-572, 1975.